1. Field of the Invention
This invention relates generally to a cathode ray tube apparatus and particularly is directed to a liquid cooling type cathode ray tube apparatus suitable for video projectors.
2. Description of the Prior Art
As is well known, a color video projector of, for example, three-tube type includes three cathode ray tubes which are respectively supplied with red, green and blue signals and produce red green and blue picture images, respectively. The red picture image, the green picture image and the blue picture image generated from the respective cathode ray tubes are respectively projected through optical lens systems, which are respectively placed in front of the panels of the cathode ray tubes so as to magnify them, on a picture screen on which they are composed as a color picture image. In the cathode ray tube for video projectors as described above, it is usual that a high voltage ranging from 26 KV to 30 KV is supplied to a phosphor screen for the purpose of obtaining a high luminous or bright image as compared with that of a general television receiver. In this case, to avoid the brightness of the phosphor screen from being deteriorated by the temperature increase in a phosphor screen of the cathode ray tube by the high voltage operation, a liquid cooling type cathode ray tube is employed.
The fundamental structure of the aforenoted liquid cooling type cathode ray tube is disclosed in the U.S. patent application Ser. No. 156,204 of the same assignee and now abandoned. Its practical example is shown in FIGS. 1 and 2.
As shown in FIGS. 1 and 2 in front of a first glass panel on which a phosphor screen 6 is formed, on a phosphor panel portion 2 of a cathode ray tube envelope, there is located a second glass panel, or front panel portion 1 with a spacer 3 therebetween. A liquid coolant 4 is sealed in the space formed between the panel portions 1 and 2. As the liquid coolant 4, a mixture of, for example, ethylene glycol and water may be used. The spacer 3 is formed as a frame-shape by a die casting process of, for example, aluminium and is sealed between the panel portions 1 and 2 by a resinuous bonding layer 5 in liquid-tight relationship. The spacer 3 serves as a heat radiation plate contacted with the liquid coolant 4 to radiate the heat generated in the liquid coolant 4, and also as an attaching plate securing for the cathode ray tube to a predetermined cabinet therein. In the figures, numeral 7 denotes a funnel portion connected to a neck portion 10 in which an electron gun 11 is provided, and 8 denotes a frit glass by which the funnel portion 7 and the phosphor panel portion 2 are sealed in air-tight relationship. In this liquid cooling type cathode ray tube, even if the temperature of the phosphor screen 6 becomes high by the scanning of a high voltage electron beam, the heat is transmitted or conducted through the phosphor panel portion 2 to the liquid coolant 4 and then radiated through the spacer 3 or radiated through the front panel portion 1 so that the increase of the temperature at the phosphor screen 6 is suppressed and deterioration of the brightness of the phosphor screen is avoided.
In the conventional liquid cooling type cathode ray tube as a sealing member for both the panel portions 1 and 2, particularly the resinuous bonding layer 5, a silicone bonding agent of, for example, thermosetting type is utilized. Such silicone bonding agent is generally white. Therefore, as shown in FIG. 2, the scattered light emitted from the phosphor screen 6 is reflected on the surface of the resinuous bonding layer 5 and surface of the spacer 3 as undesired reflected light 9.
Thus, when the conventional liquid cooling type cathode ray tube is used for a video projector, such undesired reflected light 9 lowers the contrast of the video picture image at its peripheral portion projected onto the picture screen of the video projector. Particularly, in a so-called rear-projector or rear-projection type projector, due to the design of the optical lens system, an ineffective picture image with other than the phosphor screen image and including sealing-member-like shadows is projected at the peripheral portion of the video picture image on the screen. Thus, in the rear-projection type projector, not only the contrast at the peripheral portion of the video picture image is lowered, but also the video picture image is hard to see due to the undesired picture image.